THE EVOLUTION OF THE BRAIN III 



in complexity hand in hand with the growing central organs. 

 The original state from which they arose is well shown in 

 such lowly animals as the earthworm. This creature has no 

 nasal cavities, eyes, or ears and yet it responds to odors, 

 Hghts, and sounds, and keeps itself oriented to gravity. 

 All these funct ons are carried out by the receptors in 

 its skin, but its responses are such as to justify the view that 

 its nervous states have not the least relation to distance 

 reception but are akin to surface reception. This most 

 probably is the condition that characterized the ancestral 

 vertebrate. To this creature every sensory stimulation, 

 whether it was from trunk or head, partook of the nature 

 of surface reception, and was devoid of any element of 

 distance. From this state of primitive surface receptivity the 

 vertebrate with its equipment of distance organs must have 

 developed. 



The first of these distance receptors to appear in verte- 

 brates was the organ of smell, for in Amphioxus, the simplest 

 of the fishes, we have an animal with a well-developed 

 olfactory pit, but without ears or eyes, though in the deeper 

 parts of its body are the elements out of which eyes could 

 be evolved. Amphioxus swims without orientation to gravity, 

 it responds to light though it cannot be said to see, and it 

 undoubtedly senses its way more or less by means of its 

 olfactory organ. Like the celebrated Nantucket captain 

 who knew the sea by the smell of the lead, this primitive 

 fish probably scents its way about. Its brain reflects this 

 meager receptor outfit, for it is scarcely more than a slight 

 swelling of the front end of the spinal cord. 



All fishes higher than Amphioxus and all other verte- 

 brates possess ears and image-forming eyes. The evidence 

 from the lowest of these, the lampreys, is that the eyes 

 evolved in advance of the ears because the ears, entirely 

 absent from Amphioxus, exist in a very primitive state in the 

 lampreys, whereas the eyes which were already foreshadowed 

 in Amphioxus show in the lampreys evidence of high 

 differentiation. 



As a distance receptor no organ is more important to the 

 vertebrate than the image-forming eye, for by its means an 

 animal can respond not only to light, as the earthworm does. 



